Henry's law is used in gas-transfer calculations. At equilibrium the concentration of a gas dissolved in
water (or any solvent) is proportional to its partial pressure. For example, the N2 concentration is given
by

where square brackets indicate concentration (mol/L), is the partial pressure (atm), and kH has the
dimensions mol/(L-atm) (Moore 1972). For example, the dissolved N2 concentration in equilibrium with
atmospheric N2 at 25ºC is

Henry's law is only valid for low dissolved gas concentrations. However, the atmospheric partial
pressures of N2, NH3, N2O, NO, NO2, and HNO2 are all low enough that Henry's law applies.

Most Henry's law constants have been determined at 25ºC, but they decrease with increasing
temperature (Figure 1). However, for the temperatures of most natural systems, the value of kH at 25ºC
is off by no more than a factor of two.

Figure 1. Henry's law constants for NO and N2 from 0 to 50ºC (Data from Dean 1979)